i'd go in the post office with guns blazing, just in case it's a setup. shoot the first person you see in the face to make an example to the others that you're not messing around. then drop your smoke bomb over the other side of the counter that you learned to make from "the anarchists cookbook" (yes, the book is real and yes, it shows how to make smoke bombs amongst numerous other things).
after smoking out the room, hop the counter and have an employee of the office bring you to your package at gunpoint. if they say they don't know where your package is, shoot them immediately and move onto the next person. don't waste any time with someone playing stupid, just shoot and move on. make sure to shoot them in the chest rather than the head so as to instill terror in the other employees from the screaming. eventually someone will make the effort to get you your package to stop the bloodshed.
after you have your package and hop back over the counter, drop the time-bomb over the counter that you learned to make from your good ole "anarchist cookbook". have it set at a 10 sec delay to give you time to get out but not enough time for the smoke to clear and for them to find out they're about to become meat sandwiches. make sure the explosive will be adequate enough to take out all the witnesses within the building. possibly bring two explosives in case one ends up being insufficient.
this should all have been accomplished within under 2 mins., giving you plenty of time to make your escape. there is obviously a bit more involved here like preventing escape through the back door and alternatives in case cops do happen to show up before you make your escape. however, i think you can take the rest from here. well, hope this helps.
